In a wireless communication system or wireless communication network, e.g. based on Wideband Code Division Multiple Access, WCDMA, the Radio Network, RN, typically comprises of one or more Radio Network Controllers, RNCs, each connected to one or more Radio Base Stations, RBSs, where the latter are responsible for transmitting data over the air interface. For High Speed Downlink Packet Access, HSDPA, packet data is sent via a Transport Network, TN, between these nodes. For HSDPA the TN is either Asynchronous Transfer Mode, ATM, or Internet Protocol, IP, based.
In the latter case, there may be a chance that the ordering of the packets received at the destination is different from that which the sender generated. This means that data arrives Out Of Sequence, OOS, in the RBS.
In self-built or leased backhaul, an operator has good control over the transport solution and OOS may not be expected. However, this is not the case in Heterogeneous Network, Hetnet, deployments where the operator might use public Internet Service Provider, ISP, network(s) for connecting RBSs.
There are different causes for OOS like packet-level load sharing, route fluttering, router forwarding lulls, parallel processing in routers, improper configuration, and/or faulty software, etc.
Packet-level load sharing: if links have same link weights to the destination, a router equally splits the traffic among these routers.
Route fluttering: forwarding path to a certain destination oscillates among a set of available routes to that destination.
Router forwarding lulls: some routers may pause its forwarding activity for buffered packets, e.g. when they process a routing update. These buffered packets are interspersed with new arrivals, thus causing packet reordering.
If data is received OOS in RBS and no attempt to correct this is done before data is further processed or conveyed to higher layers in a User Equipment, UE, then a number of negative impacts may be expected. These are OOS delivery of messages to higher layers in UE, unnecessary Radio Link Control, RLC, Acknowledged Mode, AM, retransmissions and erroneous decisions for TN Flow Control, FC.